Improving Saxford’s Air Quality with location data
This request is also available as machine-readable data.
What’s being collected?
To run its Air Quality Hotspots research, Saxford City Council will request the following mobile phone location data:
- anonymised cell tower triangulation information from Morlyn Mobile
- anonymised WiFi hotspot observations from Cabler Telecom
Location data will be requested for a constrained area of Saxford in a minimised form for the period of March 1st to 30th. The minimisation should limit the impact on the privacy of any individual.
Background on cell tower triangulation
When you carry a mobile phone it periodically communicates with nearby radio masts called cell towers. Phone companies such as Morlyn Mobile keep records of the communication between their cell towers and mobile devices.
By analysing these records it is possible to triangulate the signal strength and timing information from several cell towers and calculate an approximate location of the handset at a given time.
The accuracy varies from around ???m in a dense city centre to around 800m in a rural setting. The accuracy improves with the number of cell towers in range.
WiFi hotspot observations
WiFi hotspots are internet routers like you might have in your home, with a range of up to around 10 to 50 metres. Some WiFi hotspots are operated as a larger network, for example SaxfordCityWiFi.
The operators of city-wide WiFi networks, like Cabler Telecom, often record information about which phones or computers are move within range of their routers. WiFi routers can detect phones even when they don’t connect to the router as the Using location data to prioritise Air Quality improvements By collecting bulk location information, the project aims to learn two insights:
The number of people in each area of Sheffield Centre at different times through the day, linked against the times and areas with the highest air pollution. How many vehicles enter and leave the city and by which routes during the day?
When combined with open data on air quality, the Council will use the information to prioritise which areas of the city should be targetted for air quality improvement measures.
A possible air quality improvement measure would be re-routing traffic through the city, and this location data will be used to assess which routes could be adjusted.
Data will be minimised before it’s handed to Saxford Council
To reduce the risk of infringing on people’s privacy, and to ensure that the data held by the Council is not personally identifiable information, the data will be minimised in the following ways before being received.
Devices will be anonymised using randomly generated identifiers
To ensure collected location data is not personally identifiable, all identifiers (such as MSISDN, IMSI) removed and replaced with a single random pseudonym.
The allocation of pseudonyms will be carried out by Cabler Telecom and Morlyn Mobile prior to handing the data to Saxford Council.
Pseudonyms will have high entropy and will not be derived from the real identifiers.
Device pseudonyms will be changed hourly
Even when data points use only pseudonyms, it may be possible to de-anonymise users by analysing their daily pattern of location - for example spending the day in one location and the evening in another.
To minimise the possibility of re-identification based on journeys the pseudonyms will be regenerated on an hourly basis.
Location will only be recorded from 9am to 5pm
To further minimise the risk of re-identification, data will only be collected during normal working hours of 9am to 5pm. For the majority of people this will ensure data is not collected when they are at their home address.
Location accuracy will be limited outside the city centre
The final minimisation measure is to artificially reduce the precision of location data outside the city centre target areas to 800m. This makes it even less likely that a device can be located to the level of a particular home.
You can opt out of location data
You can opt out of your location data being used for Saxford’s Air Quality trial by texting “NOLOC SAXTON2018” to 84556.
Alternatively, you can opt out of all projects for a period of 12 months by texting “NOLOC ALL to 84556”.
Your phone provider must respect your decision and exclude your device from the data they provide to projects like Saxford Air Quality.